Dynamic expansion circuit



June 20, 1939. H. BARTELS El AL 2,162,875

DYNAMIC EXPANSION cmcun' Filed NOV. 27, 1936 L. fi .l

INVENTORS jjANS BARTELS AND GLlN WE ULBRICHT MIN AT'I'ORNEY PatentedJune 20, 1939 MT STATES DYNAMICTEXPANSIONCIRCUIT Hans Bartels andGiinther Ulbricht, Berlin-Germany, assignors to Telefunken Gesellschaftfiir Dralitlose Telegraphie m. b. H., Berlin, Germany, a corporation ofGermany Application November 27, 1936, Serial No. 113,021 In GermanyDecember 2, 1935' 2 Claims.

The present invention relates to circuit arrangements for automaticcontrol of the transmission ratio in amplifier stations in accordancewith frequency.

In the volume control of audio-frequency arrangements for thetransmission of music (such as modulating arrangements, talking filmreproducing arrangements or record plate reproducers) in order tocompensate for the frequency response characteristic of the ear, volumecontrol means are often used, in which the frequency responsecharacteristic of the amplifier arrangement depends on the controlmeans, i. e., on the volume of the reproduction. The dependence of thefrequency on the control arrangement is so chosen that it corresponds tothe curve of the sensitivity of the ear. As a result thereof the naturalimpression of the reproduction will be retained also in case of greatlydifferent reproduction levels, which, otherwise, would give theimpression of predominance of the low-or highfrequencies.

It was found from experience that, also, where the transmission ratio isautomatic-ally controlled, such frequency dependent control inaccordance with the curve of the sensitivity of the ear offers anadvantage. If namely at recording i. e., when compressing the intensityrange, the control is 50 carried out that the large amplitudes are notinfluenced, or but slightly influenced, as regards intensity, while thesmall amplitudes are however increased, it appears at reproduction as ifthe low frequencies were given preference. If, for instance, a bassviolin and a violin are played with the same volume in the abovedescribed control the bass violin appears to be much louder than theviolin. The entire sound picture is thus falsified.

In cables for transmission of wide frequency bands it is already knownto use devices for maintaining the level in conjunction with distortionreducers for the purpose of preventing certain frequency ranges, moreparticularly the high frequencies, from being transmitted with more ef-45 ficiency. This is accomplished in that in the control circuit of thelevel-maintaining device a distortion reducing means is inserted whichis, therefore, no longer dependent upon the natural speech, but iscontrolled by the speech already 50 controlled in amplitude inaccordance with the 7 frequency. In this way, the output level ismaintained constant without appearance of undesirable preference of thehigh frequencies.

In accordance with the invention, the desired 55 control depending onfrequency and in accordance with the sensitivity curve of the ear, isobtained with simple means in that the frequency response of theauxiliary circuit is adapted to the sensitivity curve of the car; Thiscan be accomplishedby means of a frequency-dependent coupling, orbyinserting frequency dependent elements in the transmission path of theauxiliary circuit (auxiliary amplifier). The auxiliary circuit furnishesthe'direct displacement potential required for the control, and which isa measure for the mean level of the impulses of the tone frequency. Inthe presence of sufficiently high alternating potentials, which is thecase for instance where impulses are returned from the output circuit tobe used for the control of one of the preliminary tubes (backwardcontrol), a special amplifier is not required. In these arrangements thefrequency-dependent element is arranged between the tap point of thealternating potential and the rectifier producing the direct controlpotential.

The aforementioned circuit appears on the surface to operatetheoretically correct only where individual frequencies are transmitted.In the transmission of several frequencies, in fact, the frequencyresponse of the individual frequencies can no longer beinfluencedseparately, since the direct potential depends simultaneously on allfrequencies. However, tests have proven that where several frequencesare simultaneously transmitted, the new arrangement permits of asatisfactory control dependent on frequency and conforming to thesensitivity curve of the ear.

In the drawing is shown a low frequency amplifier employing a gaincontrol circuit utilizing the invention.

The circuit arrangement in the drawing shows an audio-frequencyamplifier R, whose input electrodes are coupled across the secondary ofthe audio input transformer Te. The symbol G denotes a generator, orsource, of audio-frequency currents, and the source G is connectedacross the primary winding of the audio input transformer. The symbol R1denotes the inner resistance of G. The amplified audio output of thetube is impressed upon the audio output transformer Ta. The Volume rangeof the output of amplifier R is expanded by using the network Hincluding tube VH, and the output of the latter is impressed upon arectifier l by means of the transformer TK. The resistor 3 is connectedin series with the secondary winding of transformer TK and rectifier I,the positive side of the resistor 3 being connected to the grid of tubeR adjacent the output plate, while the opposite terminal of resistor 9is at ground potential. A condenser 2 is connected in shunt withresistor 3 and acts to by-pass audio pulsations. The condenser C is sochosen that at low frequencies (such as cycles) it is lower inresistance than the sum of R1 and RK (Rx being the coupling resistanceof the auxiliary circuit H). The constants of condenser C and resistorRK are chosen so that the expansion circuit will have a frequencyresponse characteristic corresponding to the frequencysensitivitycharacteristic of the human ear. It is well known that thefrequency-sensitivity characteristic of the human ear is such that theear is relatively more sensitive to low frequency sounds as the generallevel of intensity is increased. Of course, as the signal input fromsource G increases in amplitude, the gain of tube R will be increasedbecause of the increasing voltage drop across resistor 3, but the gainof tube R will be relatively less for low frequencies than for highfrequencies.

What is claimed is:

1. In combination with a source of audio-frequency currents, a tubeprovided with input electrodes, an output electrode, and a gain controlelectrode disposed adjacent the output electrode, an expansion circuitcomprisinga rectifier having an input circuit connected to said source,a resistor in circuit with said rectifier for developing a directcurrent voltage which increases with an increase in amplitude of currentfrom said source, means for applying said direct current voltage to saidgain control electrode in a sense to increase the tube gain, and saidrectifier input circuit including a network whose constants are chosento impart a frequency-response characteristic to the expansion circuitwhich varies with intensity inversely to the variation of thefrequency-sensitivity curve of the ear.

2. In combination with a source of audio-frequency currents, a tubeprovided with input electrodes, an output electrode, and a gain controlelectrode disposed adjacent the output electrode, an expansion circuitcomprising a rectifier having an input circuit connected to said source,a resistor in circuit with said rectifier for developing a directcurrent voltage which increases with an increase in amplitude of currentfrom said source, means for applying said direct current voltage to saidgain control electrode in a sense to increase the tube gain, and saidrectifier input circuit including a network whose constants are chosento impart a frequency-response characteristic to the expansion circuitwhich varies with intensity inversely to the variation of thefrequency-sensitivity curve of the ear, and said rectifier input circuitadditionally including an amplifier disposed between said network andsaid rectifier.

HQNS BARTELS. GUNTHER U'LBRICHT.

